1. Field of the Invention
The present invention relates generally to wireless digital communication systems. More specifically, the invention relates to a system and method for dynamically biasing a radio frequency (RF) power amplifier used for transmitting wireless communications.
2. Description of the Prior Art
A digital communication system typically transmits information or data using a continuous frequency carrier with modulation techniques that vary its amplitude, frequency or phase. After modulation, the signal is amplified and transmitted over a communication medium.
A multiple-access communication system allows a plurality of subscriber units to access the same communication medium to transmit or receive information. The communication medium is commonly referred to as the communication channel, conveying information from one location to another. For RF communications, the channel is the electromagnetic frequency spectrum which extends from very low frequencies of several kHz, through the short waves of several MHz, to very high frequencies and ultrahigh frequencies extending up to several hundred MHz and into the microwave region beginning approximately at 1 GHz.
A prior art multiple access communication system is shown in FIG. 1. Communication techniques such as frequency division multiple access (FDMA), time division multiple access (TDMA), carrier sense multiple access (CSMA), code division multiple access (CDMA) and others allow access to the same communication medium for more than one subscriber unit. These techniques can be mixed together creating hybrid varieties of multiple access schemes. For example, time division duplex (TDD) mode of the proposed 3rd generation wireless protocols is a combination of TDMA and CDMA.
An example prior art CDMA wireless communication system is shown in FIG. 2. The communication data is transmitted with a broadened band (spread spectrum) by modulating the data to be transmitted with a pseudo-noise (PN) signal. The data signal to be transmitted may have a bandwidth of only a few thousand Hertz distributed over a frequency band that may be several million Hertz. The communication channel is used simultaneously by a plurality of independent subchannels. For each subchannel, all other subchannels appear as interference.
As shown, a single subchannel of a given bandwidth is mixed with a unique spreading code which repeats a predetermined pattern generated by a wide bandwidth PN sequence generator. These unique spreading codes are typically pseudo-orthogonal to one another such that the cross-correlation between the spreading codes is close to zero. A data signal is modulated with the PN sequence to produce a digital spread spectrum signal. A carrier signal is then modulated with the digital spread spectrum signal and transmitted. A receiver demodulates the transmission to extract the digital spread spectrum signal. The transmitted data is reproduced after correlation with the matching PN sequence. When the spreading codes are orthogonal to one another, the received signal can be correlated with a particular subscriber unit signal related to the particular spreading code such that only the desired subscriber unit signal related to the particular spreading code is enhanced, while the other signals for all other subscriber units are not enhanced.
Since many subchannels in a CDMA system share the same bandwidth, most prior art wireless communication systems use some form of adaptive transmit power control (TPC) to prevent one subchannel from jamming another. When a subscriber unit or a base station is receiving a specific signal, all other subchannels or subscriber units"" signals appear as noise. Therefore, increasing the power level of one subscriber unit""s signal increases the noise presented to all other subscriber units.
In prior art CDMA communication systems, a base station transmits a communication signal on a downlink to a particular subscriber unit. Upon reception, a qualitative signal measurement is taken and compared. Based on the comparison, a TPC signal is sent in an uplink to the base station, ordering the base station to either increase or decrease its transmit power to that particular subscriber unit. This methodology is known as forward channel power control. Conversely, power control for the transmissions sent from a subscriber unit to the base station is known as reverse channel power control.
The power level of a signal output for transmission is affected by adjusting the signal amplitude input to an RF amplifier with the TPC signal using a pre-driver stage, a variable gain amplifier, an attenuator or the like. However, the gain and bias of the amplifier remain fixed. Therefore, while the transmitted signal amplitude is increased or decreased, the operating point of the amplifier is constant.
The proposed 3rd generation wireless protocols provide wide bandwidth, high data rate communication. The proposed bandwidths are a 5 to 10 MHz communication channel. However, it is known that approximately 10 to 15 dB of fast fading occurs. For example, if a mobile subscriber unit is located at the border of a defined cell and is transmitting at maximum power, a 10 to 15 dB transmit output power margin is required for momentary increments of time. Such a condition is graphically represented in FIG. 3 by a plot of subscriber unit output power in dB versus time in seconds. The average transmit output power ranges between 12 and 17 dB. The occurrence of transient peaks above average transmit power amounts to approximately one (1) to ten (10)% across the sixteen (16) second time distribution sample shown in FIG. 3. This demonstrates the limited duration for which high transmit power is required.
The most common method for modulating data signals is quadrature amplitude modulation (QAM) which varies a predefined carrier frequency amplitude and phase according to an input signal. The reason for the popularity is the many types of QAM (64QAM, 256QAM, etc.) and quadrature phase shift keying (QPSK) use the available bandwidth more efficiently by including amplitude information as part of the modulation unlike frequency modulation (FM), frequency shift keying (FSK), phase shift keying (PSK), or binary phase shift keying (BPSK) that contain little or no amplitude information. In order to amplify the signal properly, the transmitter power amplifier must operate in a linear mode. The dynamic range of input signals at the modulator port can be very large. For example, in 3rd generation wireless protocols, the input signal peak to average ratio may be greater than 10 dB.
Large transient peaks are undesirable. For every 3 dB increase in transmit output power, twice the base RF amplification power in Watts is required, which may force the amplifier into a nonlinear operating region of its response curve. This results in increased out-of-band emissions and reduced amplifier efficiency. In addition, the amplifier""s power source must have a capacity greater than the maximum transient that may be expected. This is particularly undesirable in hand-held battery operated devices. To design for higher power levels resulting from high transients, more complex amplifier circuitry is required. Otherwise, compromises between amplifier gain, battery life and communication time will result.
The prior art discloses many techniques to increase the efficiency of RF power amplifiers such as pre-distortion generators, envelope feedback correction and feed forward error correction. However, the remedies used in the prior art to increase RF power amplifier efficiency exacerbate existing design problems.
Accordingly, there exists a need for an RF amplifier that addresses the problems associated with the prior art.
The present invention is a system and method for dynamically adjusting the operating bias of an RF transmitter amplifier in response to a transmit power control (TPC) signal. The invention provides high-efficiency RF power amplification for applications where the available power source is limited. The invention uses a TPC signal resident in certain communication architectures with a detector and voltage-to-current converter to arrive at an amplifier operating bias in direct correspondence with transmitting power demand.
Accordingly, it is an object of the present invention to dynamically adjust the operating bias of an RF amplifier in dependence upon signal amplification demand.
Other objects and advantages of the system and method will become apparent to those skilled in the art after reading the detailed description of the preferred embodiment.